Polyacrylates are widely used for high-grade industrial applications, as adhesives, more particularly as pressure-sensitive adhesives or heat-sealing adhesives, having proved to be highly suitable for the growing requirements in these areas of application. For instance, pressure-sensitive adhesives (PSAs) are required to have a good initial tack, but also to meet exacting requirements in terms of shear strength. At the same time these compositions must be suitable for coating onto carrier materials. All of this can be achieved through the use of polyacrylates with a high molecular weight and high polarity, and their efficient crosslinking. Moreover, polyacrylates can be produced to be transparent and stable to weathering.
For the coating of polyacrylate compositions useful as PSA from solution or as a dispersion, thermal crosslinking has long been state of the art. In general, the thermal crosslinker—for example a polyfunctional isocyanate, a metal chelate or a polyfunctional epoxide—is added to the solution or dispersion of a polyacrylate equipped with corresponding functional groups, the resulting composition is coated as a sheetlike film onto a substrate, and the coating is subsequently dried. Through this procedure, organic solvents, or water in the case of dispersions, are evaporated, and the polyacrylate, accordingly, is crosslinked. Crosslinking is very important for the coatings, since it gives them sufficient cohesion and thermal shear strength. Without crosslinking, the coatings would be too soft and would flow away under even a low load. Critical to a good coating outcome is the observance of the potlife (processing life, within which the system is in a processable state), which can vary greatly according to crosslinking system. If this life is too short, the crosslinker has already undergone reaction in the polyacrylate solution; the solution is already partly crosslinked and can no longer be applied uniformly.
The technological operation for producing PSAs is in a state of continual onward development. Motivated by more restrictive environmental impositions and by rising prices for solvents, an aim is to eliminate the solvents as far as possible from the manufacturing operation. Within the industry, therefore, there is continual growth in the importance of melt processes (also referred to as hotmelt processes) with solvent-free coating technology for the production of PSAs. In such processes, meltable polymer compositions, i.e. polymer compositions which at elevated temperatures enter into the fluid state without decomposing, are processed. Such compositions can be processed outstandingly from the melt state. In ongoing developments of this procedure, an aim is to make the production of the product compositions as well a low-solvent or solvent-free operation.
The introduction of the hotmelt technology is imposing growing requirements on the adhesives. Meltable polyacrylate compositions in particular (synonymous designations: “polyacrylate hotmelts”, “acrylate hotmelts”) are being investigated very intensively for improvements. In the coating of polyacrylate compositions from the melt, thermal crosslinking has to date not been very widespread, in spite of the potential advantages of this method.
Acrylate hotmelts have to date been crosslinked primarily through radiation-chemical methods (UV irradiation, EBC irradiation). This procedure, however, is associated with a variety of disadvantages:                In the case of crosslinking by means of UV rays, only UV-transparent (UV-pervious) layers can be crosslinked.        In the case of crosslinking with electron beams (electron beam crosslinking or electron beam curing, also EBC), the electron beams possess only a limited depth of penetration, which is dependent on the density of the irradiated material and on the accelerator voltage.        In both of the aforementioned methods, the layers after crosslinking have a crosslinking profile, and the pressure-sensitive adhesive layer does not crosslink homogeneously.        
The pressure-sensitive adhesive layer must be relatively thin in order for well-crosslinked layers to be obtainable by chemical radiation methods. The thickness through which radiation can pass, though indeed varying as a function of density, accelerator voltage (EBC) and/or active wavelength (UV), is always greatly limited; accordingly, it is not possible to effect crosslinking through layers of arbitrary thickness, and certainly not homogeneously.
Also known in the prior art are a number of processes for the thermal crosslinking of acrylate hotmelts. In each of these processes a crosslinker is added to the acrylate melt prior to coating, and then the composition is shaped and wound to form a roll.
Direct thermal crosslinking of acrylate hotmelt compositions comprising NCO-reactive groups is described in EP 0 752 435 A1. The isocyanates used, which are free from blocking agents and are, more particularly, sterically hindered and dimerised isocyanates, require very drastic crosslinking conditions, and so a rational technical implementation presents problems. Under the kind of conditions which prevail on processing from the melt, the procedure described in EP 0 752 435 A1 leads to rapid and relatively extensive crosslinking, and so coating of the composition onto carrier materials is difficult. In particular it is not possible to obtain homogeneous layers of adhesive of the kind that are needed for many technical applications of adhesive tapes.
Also prior art is the use of blocked isocyanates. A disadvantage of this approach is the release of blocking groups or fragments which may have an adverse effect on the technical adhesive properties. One example is U.S. Pat. No. 4,524,104 A. It describes pressure-sensitive acrylate hotmelt adhesives which can be crosslinked using blocked polyisocyanates together with cycloamidines or salts thereof as catalyst. With this system, the required catalyst, but especially substances produced such as HCN, phenol, caprolactam or the like, may severely impair the product properties. With this approach, moreover, there is a need often for drastic conditions in order to release the reactive groups. Significant application of this approach is so far unknown and appears, furthermore, to be unattractive.
DE 10 2004 044 086 A1 describes a process for the thermal crosslinking of acrylate hotmelts that coats a solvent-free functionalized acrylate copolymer, which following metered addition of a thermally reactive crosslinker has a processing life that is long enough for compounding, conveying and coating, onto a web-form layer of a further material and then crosslinks this coating under mild conditions until the cohesion achieved is sufficient for pressure-sensitive adhesive tapes. A disadvantage of this process is that the reactivity of the crosslinker (isocyanate) predetermines the free processing life and the degree of crosslinking. Isocyanate crosslinkers react in some cases even during their addition; consequently, depending on the system, the gel-free time can be very short. A composition with a sizable fraction of functional groups such as hydroxyl groups or carboxylic acid can in that case no longer be applied sufficiently well. A streaky coat interspersed with gel specks and hence inhomogeneous would be the result. Another problem which arises is that the achievable degree of crosslinking is limited. If a higher degree of crosslinking through addition of a higher quantity of crosslinker is desired, this has disadvantages when polyfunctional isocyanates are used. The composition would react too quickly and would be coatable, if at all, only with a very short processing life and hence at very high coating speeds, which would exacerbate the problems of the non-homogeneous coating appearance.
Crosslinking by means of polyfunctional epoxides is described in EP 1 978 069 A1, it having been shown that through the use of accelerators, without which the epoxides would undergo, to all intents and purposes, no reaction with the carboxyl groups present in the polymer, the degree of crosslinking can be adjusted independently of the crosslinking kinetics. In order to make sure that the composition is coatable after melt processing, crosslinking in the extruder must take place only to a small extent and must subsequently continue at temperatures lower than in the extruder, in order for ideal product properties to be achieved. While the crosslinker-accelerator systems described in EP 1 978 069 A1 do meet this requirement and can be used industrially, the secondary crosslinking at room temperature was too slow. Secondary crosslinking at elevated temperatures is frequently undesirable if the PSAs have already been wound up into rolls, which may lose their shape as a result of the heat-treatment steps.
Epoxides react fundamentally only under the influence of heat, and more particularly only after prolonged supply of thermal energy. Known accelerator substances such as ZnCl2, for example, do lead to an improvement in the reaction capacity within the temperature range of polymer melts, but in the absence of a supply of thermal energy from the outside (in other words, for example, at room temperature), the reactivity of the epoxides is lost, even in the presence of the accelerators, and so the crosslinking reaction breaks down (in other words, at the prevailing temperature, the accelerators no longer have an accelerating activity). This is a problem especially when the polyacrylates processed as a hotmelt are coated within relatively short time periods (several minutes) and then, in the absence of a further supply of heat, cooled rapidly down to room temperature or storage temperature. Without initiation of a further crosslinking reaction it would not be possible to achieve high degrees of crosslinking, and for numerous fields of application of polyacrylates, such as their use as PSAs in particular, this would have the very deleterious consequence of inadequate cohesion of the composition.
If the crosslinker system, with only thermally functioning accelerators, such as ZnCl2, were to be introduced too early into the polyacrylate system (in order to achieve a sufficient degree of crosslinking), then the compositions would no longer be able to be homogeneously processed, more particularly compounded and coated, since they would crosslink too quickly or would even “gel” (undergo uncontrolled crosslinking). If, on the other hand, the accelerator causes too little activation of epoxide crosslinking, then a very long secondary crosslinking time can be expected or the compositions will have to be stored at high temperatures, which is undesirable.
It is an object of the present invention to enable thermal crosslinking of polyacrylate compositions which can be processed from the melt (“polacrylate hotmelts”), the intention being that there should be a sufficiently long processing life (“potlife”) available for processing from the melt, especially as compared with the potlife in the case of the known thermal crosslinking systems for polyacrylate hotmelts. It ought at the same time to be possible to do without the use of protective groups, which would have to be removed again possibly by actinic radiation or other methods. Moreover, the intention is that it should be possible to set the degree of crosslinking of the polyacrylate composition to a desired level, without adversely affecting the advantages of the operating regime. Even at low temperatures, the secondary crosslinking is to proceed rapidly to an end level.
In the text below, the polyacrylate compositions are also referred to synonymously for short as “polyacrylates”. For the noncrosslinked polyacrylate compositions, the term “polymerisates” is also used, while the term “polymers” is used for the fully or partly crosslinked polyacrylate compositions.